1. Field of the Invention
The present invention relates to a semiconductor laser diode and a method of producing the same. The invention particularly relates to a semiconductor laser diode which operates at a low threshold current level and a method of producing the same of vapor phase epitaxy (VPE) method by which a layer of higher uniformity is formed and a thin film is controlled better.
2. Discussion of the Related Art
FIG. 9 shows a conventional semiconductor laser diode which is published in Electronic Let. in 1981. In this semiconductor laser diode, a current block layer 2 of p-type InP is formed on a substrate 1 of n-type InP. The current block layer 2 and the substrate 1 are removed by chemical etching in the vertical direction so that a tip of a V groove can arrive at a surface of the substrate 1. As a result, the V groove 3 is formed and a crystal surface is exposed. Also, a clad layer-4 of n-type InP and an active layer 5 of p-type GaInAsP are formed on the current block layer 2 and at the bottom of the V groove 3, and a clad layer 6 of p-type InP is formed thereon. Moreover, a contact layer 7 of p-type GaInAsP is formed on the clad layer 6 and electrodes 8 and 9 are formed on the each surface of the contact layer 7 and the substrate 1.
In this semiconductor diode, the active layer 5 is buried in the groove 3, and a current concentrates to the active layer 5 of the groove 3 by pn reverse bias junction formed on the both sides of the groove 3. Therefore this semiconductor operates at a low threshold current level of 8-20 mA.
Furthermore, a SBA (Self aligned laser with Bent Active layer) laser is another conventional semiconductor laser diode is produced by MOVPE method. FIG. 10 shows a laser diode which is published in the spring syposium of applied physics society in 1985. In this SBA laser, a first clad layer 11 of p-type AlGaAs and a current block layer 12 of n-type GaAs are formed on a substrate 10 of p-type GaAs. A groove 13 of which a section is a reverse trapezoid is formed in this current block layer 12 and then the first clad layer 11 is exposed at the bottom of this groove 13. A second clad layer 14 of p-type AlGaAs, an active layer 15 of p-type, n-type or undoped AlGaAs, a clad layer 16 of n-type AlGaAs and a contact layer 17 of n-type GaAs are formed on the first clad layer 11 and the current block layer 12. Electrodes 18 and 19 are formed on the each surface of the contact layer 17 and the substrate 11.
In this SBA laser diode, a current flows through an opening portion of the stripe portion 13 of the current block layer 12. In the active layer 15, a portion 20 which is above the opening portion of the groove 13 is an active portion. Also, by MOVPE method, the active layer 15 can be flexed to be a near form of groove 13. In this flexed portion, the difference of refractive index in the horizontal direction occurs and optical confinement in this direction can be achieved.
In the semiconductor laser diode shown in FIG. 9, as the active layer 5 is buried in the B groove 3, LPE method is used as a second growth method after forming the V groove 3. However, in LPE method, it is difficult to form a thin film and a thickness of a layer uniformly.
In the SBA laser shown in FIG. 10, current blocking is executed by pnpn-type reverse bias junction of the first clad layer 11, the current block layer 12, the second clad layer 14 and the clad layer 16, outside the groove 13 of reverse trapezoid. The second clad layer 14 requires enough thickness of approximate 1 .mu.m so that a depletion layer cannot spread in whole layers and a punch through cannot occur when impressing reverse bias voltage. Moreover, as a carrier density of the second clad layer 14 is raised to approximate 10.sup.18 /cm.sup.3 so that it can be effective as a clad layer, a current spreads in the second clad 14 in the parallel direction to a junction plane. Therefore a problem that the laser diode cannot operate at a low current threshold level occurs.